Nearly all magnetic recording media have a surface layer consisting essentially of magnetizable particles dispersed in nonmagnetizable binder. To permit recovery and reuse of solvents from which the binder is coated, the solvents should be capable of repeated distillation without chemical change. In this respect, aromatic solvents such as toluene and xylene are superior to the ketones. Ease of solvent recovery is enhanced if only one solvent is present, but it has been difficult to obtain usefully smooth dispersions of magnetic pigment when using only one solvent with some of the binders of the prior art.
A widely used binder material is phenoxy resin which is a high molecular weight thermoplastic copolymer of equivalent amounts of bisphenol A and the diglycidyl ether of bisphenol A such as is sold by Union Carbide Corporation as PKHH. To provide flexibility, the phenoxy resin may be blended with polyurethane. To provide good wear resistance and resistance to blocking at elevated temperatures, the binder polymers are crosslinked, primarily during the drying step, but the crosslinking reactions tend to continue for some time after the tape or other medium has cooled to ambient temperature.
The crosslinking involves a number of problems. First, there is the problem of maintaining adequate pot-life. Second, it is desirable that the polishing step, to which most recording tapes are subjected, be carried out after the coating has been dried out before the binder becomes fully crosslinked. The proper timing can be difficult to control. Furthermore, difficulties in slitting are sometimes encountered after a phenoxy-urethane binder has fully cured.
Even though crosslinking involves problems, magnetic recording tapes of the prior art employing uncrosslinked binders, such as vinyl resins, have not provided equivalent resistance to wear. Such tapes have also exhibited poor resistance to blocking after exposure to high temperatures, especially if polished to the high degree of smoothness required for many applications.